1. Field of the Invention
The present invention relates to a starting apparatus of a passenger protecting apparatus such as air bag or seat belt pretensioner, which is actuated by detecting collision of a vehicle.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional air bag starting control unit which is disclosed in, for example, Japanese Patent Application Publication No. 59-8574. In FIG. 1, reference numeral 1 is an acceleration sensor (hereafter abbreviated as G sensor), 155 is an integrating circuit for integrating an acceleration signal output from the G sensor 1, and 156, 159 are comparator circuits for comparing an output from the integrating circuit 155 with starting prediction levels V.sub.1 and V.sub.2. Further, reference numeral 157 is a time constant circuit including a diode D.sub.2, a resistor R.sub.2 and a capacitor C.sub.2, and 158 is a comparator circuit for comparing an output from the time constant circuit 157 with the output from the integrating circuit 155. Reference numeral 160 is a reset pulse oscillator receiving an output from the comparator 159 as an input, 161 is a differentiating circuit including a capacitor C.sub.1 and a resistor R.sub.1, and D.sub.1 is a diode for supply an output from the differentiating circuit 161 to an input terminal of the integrating circuit 155.
A description will now be given of the operation of the prior art apparatus. At a time of collision of a vehicle, the G sensor 1 converts acceleration into an electrical acceleration signal, and the acceleration signal is integrated and converted into a speed signal by the integrating circuit 155. Typically, the integrating circuit 155 is reset for each predetermined cycle by an output signal from the pulse oscillator 160, and a starting signal is output if the output from the integrating circuit 155 exceeds the starting prediction level of the comparator circuit 158.
However, if the output from the integrating circuit 155 exceeds the starting prediction level of the comparator circuit 159, the output drops so as to extend the cycle of the pulse oscillator 160, and passes through the differentiating circuit 161, resulting in an extended cycle of reset pulse for the integrating circuit 155.
Further, the output from the integrating circuit 155 varies the starting prediction level of the comparator circuit 158 through the comparator circuit 156 and the time constant circuit 157. In case the output from the integrating circuit 155 exceeds the starting prediction level, the comparator circuit 158 outputs a starting signal so as to avoid malfunction when receiving impact which has no need to start the starting apparatus of the passenger protecting apparatus.
The starting apparatus of the passenger protecting apparatus in the prior art is provided as set forth above. Accordingly, there are some drawbacks in that a circuit for generating a trigger is required to reset the integrating circuit within a predetermined period, and a time delay generated by the trigger circuit causes another time delay required to generate the starting signal. Further, there is another drawback in that the passenger protecting apparatus may be unnecessarily started by, for example, rough road travelling (i.e., travelling other than the collision such as running onto a curb) due to indistinctness in a difference between the rough road travelling and head-on collision at a low speed if the starting apparatus is started by only the acceleration signal at the time of the collision of the vehicle. There is still another disadvantage of difficulty of discriminating between the head-on collision at the low speed (i.e., collision having no need to start the passenger protecting apparatus) and special collision (i.e., collision requiring starting of the passenger protecting apparatus) such as slipping under a rear deck of a truck.